This invention relates to magnetic flowmeters and more particularly to isolation amplifiers and ranging circuits especially suited for such flowmeters and similar applications.
Certain apparatus such as fluid flowmeters produce an analog output signal which must be processed or converted before it can be used externally of the apparatus. For example, in the case of a fluid flowmeter, the analog signal is indicative of flow rate. In a magnetic flowmeter a magnetic field across a flowtube generates a voltage in the fluid which is proportional to the flow rate. This voltage is sensed by a pair of electrodes in contact with the fluid and amplified by a signal processing system. More particularly, the analog output of the electrodes in these systems must be isolated by an isolation barrier from the external measuring equipment so as to maintain a true reading of flow rate.
In certain magnetic flowmeters, the analog signal output is converted to a digital signal, such as a train of pulses, the frequency of which is proportional to flow rate. Such a system is shown in U.S. Pat. No. 4,309,909 to Grebe, Jr. et al. entitled Crystal Stabilized Voltage to Frequency Converter with Digital Calibration for Flowmeters. Such systems work well for their intended purposes, but they could be improved. For example, such prior systems are believed to require high quality components to maintain the accuracy associated with such instrumentation. Such components may include crystal oscillators and counter/timers on both sides of the barrier.
In certain applications, the output of the flowmeter is supplied to external equipment which requires a certain range of signal from the flowmeter. Heretofore, the analog signal representative of flow rate could be scaled as follows. The analog signal would be converted by a voltage/frequency converter to produce a digital signal which was then scaled by a rate-multiplier circuit. The output of the rate multiplier was converted back to an analog signal using a frequency/voltage converter. Each conversion between analog signal and frequency has an error associated with it however. An alternative scheme would be to use a digital-to-analog converter (DAC) to scale the analog signal, but binary-coded-decimal (BCD) type DACs are not believed to be available at reasonable cost to more than three and one-half decimal places.